As a prior art example of such a color image forming apparatus, an outline of the structure of the color printer described in Publication of Unexamined Patent Application (Tokkai) No. Hei 7-36246 is explained. As can be seen in FIG. 8, which shows the internal structure viewed from the side, this apparatus comprises an intermediate transfer belt unit 201 including a transfer belt 202, a primary transfer roller 203, a secondary transfer roller 204, a cleaner roller 205, and a waste toner reservoir 206. Composition of color toner images is performed on the transfer belt 202.
Four image forming units 207Bk, 207Y, 207M and 207C for black, yellow, magenta and cyan, each unit being of sector shape in cross section, are arranged circularly in the middle of the printer to form a group of image forming units 208. When an image forming unit 207Bk, 207Y, 207M or 207C is set properly in the printer, mechanical and electrical connection systems are established between one of the image forming units 207Bk, 207Y, 207M and 207C and the machine body side via mutual coupling members, so that both sides are mechanically and electrically connected.
The image forming units 207Bk, 207Y, 207M and 207C are supported by a supporter, which can revolve around a cylindrical shaft 209, and are collectively rotated by a motor. Each image forming unit is successively moved by rotation to an image forming position 210, where it opposes the primary transfer roller 203 spanning the intermediate transfer belt 202. The image forming position 210 is also the exposure position for exposure by a laser beam 211.
Numeral 212 is a laser exposing device arranged in the lower part of the printer. The laser signal beam 211 passes through a light path opening 213 between the image forming units 207M and 207C, and through an opening provided in the cylindrical shaft 209, and enters a mirror 214, which is positioned inside the shaft 209 and fixed directly to the machine body. The laser beam 211 reflected by the mirror 214 enters the image forming unit 207Bk located at the image forming position 210 through an exposure opening 215, and passes through an exposure space between a developing device 216 and a cleaner 217, arranged on the upper and the lower side in the image forming unit, and enters an exposure portion on the left side of the photosensitive drum 218. The laser signal beam is scanned by the exposing device along the direction of the axis of the photosensitive drum 218 and a latent image is formed. The toner image is formed on the surface of the photosensitive drum 218 by development with the developing device 216.
The toner image formed on the photosensitive drum 218 is transferred to the intermediate transfer belt 202. Then, the group of image forming units 208 rotates by 90 degrees, so that the yellow image forming unit 207Y moves to the image forming position 210. An operation similar to the operation explained above for the formation of the black image is performed to form a yellow toner image overlaying the black toner image that has already been formed on the intermediate transfer belt 202. Similar operations as explained above are performed using the magenta and cyan image forming units to compose a fall color image on the intermediate transfer belt 202. This full color image is further transferred onto a recording paper by a secondary transfer roller 219, and after the image on the paper is fixed by a fixing device 220, the paper is ejected.
When the image formation of one sheet is finished, the group of image forming units 208 rotates another 90 degrees, so that the black image forming unit 207Bk returns to the image forming position 210 and can engage in the next image formation.
As is described above, in an image forming device employing a conventional rotation type group of image forming units, the operation of switching the image forming units is performed four times per image formation on one sheet. A certain time is required each time the image forming unit is switched. Namely, a switching time is necessary until the exposure with the next color can begin, which encompasses the time to rotate the group of image forming units for 90 degrees, the time needed to release and couple the driving couplers between the photosensitive drum in the image forming position and the main body driving side, the time until the photosensitive drum starts to rotate and the rotation speed is stabilized, and the time until the revolving photosensitive drum is charged to a constant level.
For the output speed per sheet, this switching time is clearly not short when it comes to multiple output of consecutive sheets and becomes a big obstacle to the acceleration of color image forming apparatus.
Moreover, in a conventional color image forming apparatus, the laser exposing device is arranged outside the group of image forming units and the laser signal beam is reflected from the mirror arranged in the middle of the group of image forming units to expose the photosensitive drum in the image forming position. With such a structure, the light path from the laser exposing device to the photosensitive drum can easily become long, so that high dimensional accuracy for the optical parts constituting the laser exposing device is necessary. Moreover, the optical system of the laser exposing device is separated into laser light source and mirror, so that it cannot be optically adjusted as a single element. Therefore, an adjustment after assembly is arduous and it is difficult to ensure the scanning precision. Moreover, for reasons of miniaturization and economy, it is desirable to make the toner capacity that can be accommodated in the image forming units as large as possible and the entire apparatus as small as possible.
Under consideration of the problems of the prior art, it is a purpose of the present invention to provide a color image forming apparatus that can perform faster image formation in the case of consecutive output of multiple pages, but whose assembly and adjustment of the apparatus is easy and which enhances miniaturization.